Hydrocarbon streams which contain sulfur compounds such as mercaptans are usually referred to as sour hydrocarbon streams. In order to render these hydrocarbon streams usable, it is common in the oil refining industry to treat these streams in one of two ways. One way is to contact the sour stream (usually a liquid stream) with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent such as air at reaction conditions in order to convert the mercaptans to disulfides, Another way to treat these sour hydrocarbon streams is to contact the hydrocarbon stream with a strongly basic solution in order to extract the mercaptans into the basic solution thereby forming mercaptides and then taking the basic solution and regenerating it by contacting it with an oxidizing catalyst, an alkaline agent, and an oxidizing agent such as air to convert the mercaptides to disulfides. The disulfides are separated from the basic solution which is recycled and used to extract more mercaptans from a fresh hydrocarbon stream. The first type of process is disclosed in a number of U.S. patents, including U.S. Pat. Nos. 3,108,081; 4,156,641; 4,913,802; 4,290,913 and 4,337,147. Processes in which the mercaptans are extracted with an alkaline solution are disclosed in U.S. Pat. Nos. 2,853,432; 2,921,020; 2,988,500 and 3,408,287.
It is also known that if the liquid hydrocarbon streams contain H.sub.2 S, that this must be removed separately before the mercaptans are either extracted and oxidized or oxidized to disulfides in one step. This separate process usually involves contacting the stream with either a basic solution or an adsorbent in order to remove the H.sub.2 S present in the hydrocarbon stream. For example, U.S. Pat. No. 4,562,300 discloses a process for extracting mercaptans from hydrocarbon distillates with an alkaline solution having from about 5 to about 50 weight percent of an alkaline. Alkaline materials include sodium hydroxide, lithium hydroxide and potassium hydroxide. These alkaline materials are all very strong bases. U.S. Pat. No. 4,666,689 again discloses contacting hydrocarbon streams with alkaline solutions such as aqueous sodium hydroxide solutions. Finally, In U.S. Pat. No. 5,354,545 it is disclosed that sulfur compounds such as H.sub.2 S are removed from a gaseous effluent. The gaseous waste stream is first contacted with a basic aqueous solution and then the aqueous solution is treated in a bioreactor thereby converting the H.sub.2 S to elemental sulfur in order to regenerate the basic solution. It is disclosed in the '545 patent that an improvement in H.sub.2 S removal is observed when the recirculating basic solution contains from 0.1 to 50 g/l of sulfur.
Applicants have developed a process whereby a liquid hydrocarbon stream containing both H.sub.2 S and mercaptans is contacted with a weakly basic solution, e.g., bicarbonate solution, to simultaneously remove H.sub.2 S and mercaptans. The basic solution which now contains H.sub.2 S and mercaptans is treated in a bioreactor where the solution is contacted with a sulfide oxidizing bacteria in the presence of oxygen. The H.sub.2 S is converted to elemental sulfur while the mercaptans are converted to disulfides. The oxygen level in the reactor is controlled in order to obtain these final products. Subsequently, the sulfur and disulfides are removed from the aqueous stream which is then recirculated and used to further extract hydrogen sulfide and mercaptans from a fresh liquid hydrocarbon stream. The treated liquid hydrocarbon stream now contains substantially less hydrogen sulfide and mercaptans, and can be used in various applications.
In comparison to U.S. Pat. No. 5,354,545 applicants have demonstrated that a weakly basic solution can be used to extract H.sub.2 S and mercaptans from a liquid hydrocarbon stream. It is also surprising that the bacteria is able to function even when small amounts of hydrocarbons are present in the aqueous stream. Finally, applicants' treated aqueous stream will contain less than 0.08 g/l of elemental sulfur.